1. Field of the Invention
This invention is a further development in the art of crystal growing by the Czochralski method, and in particular relates to the monitoring and control of the cross-sectional dimensions of a crystal rod.
2. Description of the Prior Art
The Czochralski method of crystal pulling is widely used to provide crystal rods for the semiconductor industry. In summary, the Czochralski method involves melting high-purity semiconductor material in a crucible in a nonreactive atmosphere, and maintaining the temperature of the melt at just above the freezing point. A seed crystal is dipped at a particular orientation into the melt, and is thereafter slowly raised from the melt so that liquid from the melt will adhere to the withdrawing seed crystal. As the seed crystal with its adhering material is pulled away from the melt, an elongate crystal rod can be formed. The diameter of the crystal rod so formed is a function of a number of variables. For example, the temperature of the melt in the vicinity of the liquid-solid phase boundary, i.e., adjacent the interface between the melt and the growing rod, has a major effect on the rod diameter. Changes in the melt temperature can result in significant variations in the rod diameter. Such variations in the rod diameter can result in serious and costly waste, because the rod must thereafter be trimmed and cut to produce wafer slices of uniform physical dimensions.
It is believed that the temperature of the melt is dependent upon the time rate of withdrawal of liquid from the melt. Consequently, it is important to be able to control the time rate of upward motion of the rod from the melt in response to changes in the diameter of the growing crystal rod. A number of techniques have been devised to sense changes in the magnitude of the rod diameter, and to generate signals responsive to such diametric changes. Such signals indicative of changes in the rod diameter serve to actuate means for adjusting growth conditions such as the pulling rate of the growing rod in order to compensate for any irregularities in the diameter of the rod. In general, an increase in the pulling rate will tend to decrease the rod diameter while a decrease in the pulling rate will tend to increase the rod diameter. The relationships between other parameters and the rod diameter have been determined for particular crystal pulling systems on a mainly empirical basis.
U.S. Pat. No. 3,259,467 discloses a technique for controlling the diameter of a rod being pulled from a melt by continuously monitoring the weight-to-length quotient of the rod, and by regulating a growth condition such as the pulling speed in dependence upon the weight-to-length quotient. U.S. Pat. No. 3,291,650 discloses a technique for utilizing the optically reflective property of the meniscus formed at the interface between the melt and the crystal rod being drawn therefrom to provide a signal indicative of changes in the diameter of the rod. U.S. Pat. No. 3,493,770 shows the use of a photovoltaic optical pyrometer to sense variations in the radiation emitted by the surface of the melt near the rod/melt interface as the rod diameter changes, and means for adjusting various growth conditions of the rod in response to such variations in the emitted radiation so sensed. U.S. Pat. No. 3,692,499 shows means for simultaneously monitoring the radiation intensity at a plurality of points along each of three different radius lines of the meniscus which forms at the rod/melt interface, and means for controlling growth conditions such as pulling speed in response to variations in the monitored radiation intensity.
According to techniques known to the prior art, changes in the diameter of a crystal rod being grown by the Czochralski method have been detected by optical means, and signals responsive to such diametric changes have been used to adjust the growth conditions of the rod so as to counteract the tendency of the rod diameter to change. However, with the techniques known to the prior art, many unnecessary adjustments of the crystal rod growth conditions are oftentimes made. For example, as the seed crystal draws material from the melt, the material so drawn adheres to the seed crystal in a configuration which is determined by the crystalline structure of the seed crystal. Additional material from the melt adheres to material previously drawn from the melt according to a pattern determined initially by the crystalline structure of the seed crystal and the orientation of the seed crystal with respect to the surface of the melt. The resulting elongate crystal rod which is formed as the seed crystal is withdrawn from the melt will in general not have a uniformly circular cross section. The diameter of the rod will therefore vary on any given transverse cross section through the rod, because of so-called flat spots on the growing crystal. In a crystal pulling apparatus which provides for relative rotation of the growing crystal rod and the liquid melt, such variations in the rod diameter will be detected. Unless such variations which recur with regularity with each rotation of the rod are filtered out, unnecessary adjustment signals will be generated to activate means for varying particular crystal growth conditions. In the usual case, the frequency of rotation of the growing crystal rod is not much faster than the natural frequencies of the crystal growth mechanism. Consequently, filters used to suppress the generation of such unnecessary adjustment signals are likely to cause severe attenuation of the diameter indicating signal itself or to cause unwanted phase shifts in the automatic control system for activating the means for varying the growth conditions. Such unwanted phase shifts could render control of the growth conditions extremely sluggish or unstable.